The present invention is directed to a low weight, high strength, three-dimensional space structure with nearly isotropic load-bearing characteristic for reacting fully to tensive and compressive external loads. The structure provides high bending stiffness and a high degree of torsional rigidity and may be self-sufficient or used as a core material.
The closest prior art of which applicant is aware are the above cross-referenced patents which disclose the apparatus and method for making a structural core comprising a symmetrical arrangement of generally tetrahedronal shaped internal supports for such articles of manufacture as air foils. The disclosures of these patents are herein incorporated by reference.
A more detailed description of the prior art is presented in AIAA Paper No. 74-357, based on AIAA/ASME/SAE 15th Structures, Structural Dynamics and Materials Conference, Las Vegas, Nevada/Apr. 17-19, 1974. This publication indicates that a winding process used to produce the tetrahedra elements may be halted at any point to produce either "truncated" tetrahedra or a repeated series of stacked tetrahedral elements. However, no properties or uses for "truncated" tetrahedra are stated or suggested.
It is also known to provide honeycomb-core panel structures having a pair of face sheets with the honeycomb sandwiched therebetween. These honeycomb structures provide very good tensive and compressive characteristics for distributed loads which are normal to the face sheets. However, these conventional honeycomb structures are not capable of reacting fully to forces of tension and compression which are generally parallel to the face sheets; instead, they tend to buckle or delaminate accordingly. Additionally, the core of these honeycomb structures provide only line contact for bonding of face sheets.
The instant invention provides a structural panel having plural, truncated-apex elements of various geometries protruding in one direction from a base sheet, such that the base sheet and the truncation planes of the elements provide excellent bonding areas for the addition of external face sheets by adhesives, welding, and the like. Alternatively, according to the material used in the fabrication of the panel, face sheets or the like may be attached by nails, rivets, screws, or other conventional attachment means. Two of the novel panels may be combined such that the elements of one panel are caused to interlock with those of the other panel in order to provide added internal load bearing or reacting capabilities; or, the two panels could be arranged, base sheet-to-base sheet, to present only the truncation plane surfaces for face sheet bonding. Another embodiment includes elements projecting in opposite directions from one base sheet. Still further, the combined panel arrangements could be formed in one piece.
The instant invention may have, but does not require, additional external face sheets to react to external loads. However, when face sheets are desired, large surfaces are provided by the base sheets and/or truncated-apex surfaces for ease and strength of bonding, as well as increased torsional rigidity.
Geomtries of the truncated elements may include cones and generally polyhedral-structures, with altered regular tetrahedrons (having the apexes and intersecting faces truncated) being preferred. The truncated intersecting faces are referred to herein as "quasi-faces" and act as particularly good load reacting beams when two panels are combined to form interlocking elements as mentioned above. Further, the elements of a panel may have different base-to-truncated apex heights to provide large and small elements and may be arranged on the panel so that, when two panels are combined, the large elements of opposite panels interlock with adjoining quasi-faces and the truncated apexes of the small elements of opposite panels abut to provide good surface areas for attachment of the two panels.
The element structure of the novel panel is capable of accepting local loads and distributing them throughout the rest of the panel, since the structural material is distributed precisely along the lines of maximum stress density. This provides for a nearly isotropic structure capable of fully reacting to external loads.
For storage and shipping purposes, mass produced panels may be nested, with the elements of each panel fitting into like elements of an adjacent panel.
The panels may be manufactured from any formable or moldable materials including plastics, fiber glass, concrete, cement, reclaimed and recycled materials (uses as structural or non-structural aggregates) or any combination of these materials. The materials may range in hardness from resinated paper to titanium. For instance, waste pulps and fibers and reclaimed low-grade aluminum and steels are excellent materials for manufacture of the panels. By utilizing foam (in an open or closed form of the panel) or evacuating individual closed elements to produce a thermos effect, excellent insulating properties may be incorporated in the panels.
Uses for these structural panels range from pellets, to building panels (including economical load bearing walls), to aircraft wing ribs and helicopter drive shafts, to aerospace applications. Automobile bodies, portable bridges, boat hulls, and packages (such as shipping boxes) are articles of manufacture to which this technology may be adapted also.
The aesthetics of the panels may be enhanced by exposing apertures or hollow portions, by applying various colored or variegated external face sheets, and by curvilinear shaping of the panels. To meet varying design requirements, the geometric proportions and material selection of the elements may be varied for different core stiffnesses and strengths. For instance, by varying the relative sizes of the polyhedral-shaped elements, the panels may be given complex curvatures.
Techniques of manufacture such as conventional die forming and molding and casting may be used, depending on the material used.